Saturated polyalkylenes, such as polyethylene, polypropylene, etc., display elastomeric properties which make them useful in varied applications. In particular, block copolymers of vinyl aromatics and polyalkylenes are useful due to their excellent elastomeric properties without the need to be vulcanized. However, polyalkylenes have a higher flammability than is desired for certain applications. It is therefore advantageous to prepare a polymer which retains the elastomeric properties associated with the polyalkylenes, but which has reduced flammability.
As reported in U.S. Pat. Nos. 3,576,912 and 3,686,366, both issued to Winkler, halogenation of unsaturated polymers has been carried out in the prior art for preparation of fire resistant polymers. It has been found, however, that halogenation to an extent effective in providing the desired degree of fire resistance results in a loss of a portion of the elastomeric properties of the unhalogenated polymer.
The Winkler patents further disclose the preparation of block copolymers of monovinyl arches and conjugated dienes in which the diene blocks are selectively halogenated. The Winkler patents describe the substantially exclusive halogenation of the conjugated diene polymer block (such as polybutadiene or polyisoprene), with the thermoplastic monovinyl arene blocks being substantially unaffected. The Winkler patents disclose as halogenation agents N-halomides, bromoanilides, bromoamines, hydrogen bromide and N-monohalohydantoins, or the corresponding chlorinated compounds, and reacts these compounds with ethylenic unsaturation. The Winkler processes intentionally provide for essentially no reaction in the monovinyl arene polymer blocks for the express purpose of maintaining the original physical properties of the block copolymer. Although the brominated rubber produced by Winkler's methods are useful rubbers, comparing Winkler's Example IV to the base polymer of Winkler's Example I from '366 reveals that the mid-block bromination decreases tensile strength by 14% and elongation at break by 40%. The brominated polymer was "harder" than the base polymer as indicated by a modulus which was 230% of that of the base polymer. It would be desirable to have brominated polymer which retained physical properties more like those of the base polymer.
U.S. Pat. No. 4,352,909 issued to Barda et al. discloses brominated polystyrene, a process to brominate oligomers and polymers of styrene, and the use of brominated polystyrenes as flame retardant additive for engineering thermoplastic and thermoset resin compositions. The preferred process to brominate the styrene polymers is to react the polymer with a stoichiometric excess of bromine chloride in a chlorinated hydrocarbon solvent and in the presence of a Lewis acid of a metal chloride salt. This patent discloses a process to graft nuclear bromine to styrene polymers, but does not teach or suggest that the process would selectively graft halogens to aromatic rings in polymers which contain both styrene and alkylenes.
Further, the process disclosed in '909 requires that anionically polymerized polystyrene be recovered from the polymerization system, redissolved in a halogenated solvent and contacted with the Lewis acid of the metal chloride salt catalyst. A process which does not require recovery of the polymer from the polymerization solvent would be desirable. Elimination of this process step would save considerable expense in manufacturing polymers which are halogenated by this process.
It is therefore an object of the present invention to provide a halogenated vinyl aromatic-conjugated diolefin block copolymer composition having blocks of polyvinyl aromatic containing nuclear aromatic halogen, and blocks of saturated polyalkylene.
Another object of the present invention is to provide a halogenated vinyl aromatic-conjugated diolefin block copolymer composition containing substantially all of the halogen in the vinyl aromatic blocks.
An additional object of the present invention is to provide a halogenated vinyl aromatic-conjugated diolefin block copolymer which substantially retains its elastomeric properties, but which has lower flammability than the vinyl aromatic block copolymer before halogenation.
In another aspect, it is an object of this invention to provide a process to prepare a polymer comprising vinyl aromatic and hydrogenated conjugated diolefin monomer units wherein the aromatic rings are selectively halogenated by hydrogenation of a conjugated diolefin-vinyl aromatic copolymer, and then halogenation prior to separation of the polymer from the hydrogenation catalyst and medium.